1. Field of the Invention
The present invention relates to a coating apparatus for applying a liquid to the surfaces of a flexible support sheet (referred to simply as "support", hereinunder) while carrying the support without contacting the same. More particularly, the invention is concerned with a coating apparatus for coating one or two kinds of liquid on a support such as a photosensitive photographic material while carrying the support at its surface opposite to the coated surface and allowing the same to run continuously. Still more particularly, the invention is concerned with a coating apparatus suited to continuous coating of both sides of the support.
2. Description of the Prior Art
In the conventional process for producing photosensitive photographic materials coated at both sides of a support, the support is subjected twice to the same steps, i.e. the application of the liquid, gelation of the liquid and drying, such that the application of the liquid to the second side is started after the drying of the coating film on the first side. In recent years, however, various methods have been proposed to form coating layers on both sides of the support by making the latter to pass the steps of application and drying only once, in order to attain a higher productivity. These methods are grouped into several types in one of which the application of the liquid to the second side is made after gelation of the coating film on the first side. Two methods of this type have been proposed up to now: namely, (i) a first method in which, after gelation of the film on the first side of the support, the liquid is applied to the second side while the support is carried at its first side by supporting rolls in direct contact with the latter as disclosed in Japanese Patent Publication No. 44171/1973 and (ii) a second method in which, as disclosed in Japanese Patent Publication Nos. 17853/1974 and 38737/1976, the support is suspended at its first side having the coating layer by a gas jetted from the surface of a supporting roll having a predetermined curvature during the application of the liquid to the second side of the support. The first method (i) suffers from various problems as follows. Namely, a strict management of the coating system is required to eliminate scratching or dust in or on the roll surface because the coating is made defective even by a slight scratch or dust in or on the supporting roll surface. Even when there is no scratch or dust, the coating layer is disturbed when a portion of the support having a fluctuation of the coating film thickness, e.g. the portion where the application is started, spliced portions and so forth passes the supporting roll in contact with the latter. In such a case, a part of the gelated liquid may attach to the supporting roll surface to disturb the coating layer on the following portion of the support. In the method (ii) mentioned before, unevenness of the coating film thickness in the form of step lines transverse to the longitudinal direction of the support tends to be caused by a slight fluctuation of the lift of the support above the supporting roll attributable to, for example, a change or fluctuation in the tension acting on the support. In the method shown in Japanese Patent Publication No. 17853/1974 in which the end of a coater is pressed to the surface of the support while the latter is made to float above the supporting roll surface by the gas jetted from the roll having small apertures or slits, the above-mentioned tendency is appreciable particularly at the end portions of the support. In contrast, in the method shown in Japanese Patent Publication No. 38737/1976 which employs such a roll as to adapted to carry the support at both marginal edges to keep the support substantially afloat, the above-mentioned tendency is heavy particularly in the central portion of the support.
The method disclosed in Japanese Patent Publication No. 17853/1974 in which the coating is made on both sides of the support by the method (ii) mentioned before, a defect generally referred to as "unevenness due to blowing", i.e. a fluctuation in the coating film thickness tends to be caused in the first side because of dynamic pressure of the gas impinging upon the first side for carrying the support during the application of the liquid to the second side.
On the other hand, the method disclosed in Japanese Patent Publication No. 17853/1974 involves a problem that, since the variation of the lift in the widthwise direction becomes large as the width of the support gets large, it is difficult to press the end of the coater uniformly to the support and, hence, to obtain a uniform thickness of the coating layer on the entire surface of the support. In addition, the coating film thickness is apt to be varied because no specific consideration is given to the prevention of the vibration of the support at the front and rear sides of the coater. Furthermore, it is not possible to apply the bead application method employing, for example, a slide hopper which is commonly used in the application of photosensitive photographic material, because of the direct pressing of the coater to the support.
Referring again to the method disclosed in Japanese Patent Publication No. 38737/1976, the distance between the support surface and the end of the coater at both marginal ends of the support where the latter is carried by the supporting roll is not equal to that at the other portions of the support out of contact with the supporting roll. This makes it difficult to obtain a uniform thickness of the coating layer. More practically, a coating defect in the form of longitudinal stripes tends to appear. The difference of the aforementioned distance becomes larger as the width of the support becomes greater. In the worst case, some portions of the support may not be coated by the liquid at all.
In the method disclosed in Japanese Patent Laid-Open No. 45410/1980, the position of the support at portions thereof other than both marginal ends carried by the supporting roll is determined by a delicate balance between the back pressure (T/R, T: tensile strength, R: radius of curvature of support surface) produced by the tension and the reduced pressure applied by the coater. Thus, the position of the support is changed even by a slight unbalance between these forces to cause a change in the distance between the end of the coater and the support, resulting in an uneven thickness of the coating film in the form of step lines transverse to the longitude of the support. It is quite difficult to maintain the reduced pressure applied by the coater, over the entire width of the support. In consequence, the coating defects such as unevenness of coating thickness in the form of step lines transverse to the longitudinal direction, longitudinal stripes and liquid application failure are liable to occur.
The present inventors have made an intense study to overcome the problems of the prior art heretofore described and have reached the following conclusion.
From a generic point of view, in the known methods or apparatus in which the support is carried by the gas, i.e. carried by supporting roll without contacting the same, only a fundamental construction is offered or an effort is concentrated mainly to effect the elimination of the vibration of the support in the thicknesswise direction of the latter. Namely, no specific consideration is given to the uniformalization of the gap between the coater end and the support surface (this gap will be referred to as "coater gap", hereinunder) along the width direction of the support. This is quite vital in the bead application method which employs a slide hopper or the like, because the lack of uniformity of the coater gap along the width direction is liable to cause the unevenness of the coating layer thickness in the form of longitudinal stripes. In the worst case, some portion of the support is not at all contacted by the liquid. Usually, the end of the coater and the outer surface of the gas jetting device are constructed as linearly as possible along the width direction of the support. Therefore, the coater gap will be uniformalized along the width direction of the support within the tolerance of the machining, provided that the lift of the support from the surface of the gas jetting device is uniform along the width direction of the support. As a matter of fact, however, the lift of the support is fluctuated largely along the width direction of the support because no positive measure has been taken for uniformalizing the lift. Therefore, it has been quite difficult to obtain a practical application device which can uniformly apply the liquid. The uniform lift of the support along the width direction becomes more difficult as the width of the support gets larger, and this problem becomes more serious as the width of the support is increased beyond about 500 mm. Considering that the support width usually exceeds 500 mm in most coating apparatus designed for attaining a high productivity, it is almost impossible to avert from the above-described problem in developing a non-contact carrying type coating apparatus for coating both sides of the support. Throughout the specification, the term "wide support" is used to mean support having a width exceeding 500 mm, while the term "narrow support" means support of width less than 500 mm.